1. Field
This patent specification describes a switching regulator, and more particularly, a switching regulator capable of preventing reverse current.
2. Background Art
Recently, energy saving has been actively promoted to protect the environment. For battery-powered portable equipment, such as mobile phones, digital cameras, and the like, energy efficiency is especially important to prolong battery life. Such portable equipment typically uses a switching regulator that includes an inductor because it is efficient and can be made compact.
Such switching regulator generally includes a switching transistor and a synchronous rectification transistor, and operates in a continuity mode and a discontinuity mode. In the continuity mode, a current flows through the inductor continuously. By contrast, in the discontinuity mode, the current does not flow through the inductor continuously. Consequently, when the switching regulator operates in the discontinuity mode under light load conditions, a reverse current may flow from an output terminal to the inductor. As a result, the performance efficiency of the switching regulator decreases. Further, in such switching regulator, a rush current and an overshoot in an output voltage of the switching regulator may be generated due to an overdrive at start-up.
To avoid generation of the rush current and the overshoot of the output voltage at start-up, a known switching regulator employs a soft-start circuit that can increase the output voltage gradually. More specifically, the soft-start circuit increases an on-duty cycle of a PWM (Pulse Width Modulation) pulse gradually for a predetermined period from start-up so as to prevent generation of the rush current and the overshoot of the output voltage.
To avoid the reverse current, the switching regulator generally employs a reverse current prevent circuit like that shown in FIG. 1.
In FIG. 1, the switching regulator includes a switching transistor M101, a synchronous rectification transistor M102, an inductor L101, output capacitors C101 and C102, a comparator 101, and an AND circuit 102. The switching transistor M101 and the synchronous rectification transistor M102 are NMOS transistors.
When energy stored in the inductor L101 is discharged completely while the synchronous rectification transistor M102 is on, the reverse current occurs. Namely, a charge stored in the output capacitor C102 connected between the output terminal and ground discharges to ground through the inductor L101 and the synchronous rectification transistor M102. When the reverse current occurs, a voltage VA at a connection node A between the inductor L101 and the synchronous rectification transistor M102 becomes a positive value.
The voltage VA at the connection node A is input to an inverted terminal of the comparator 101. A ground voltage Vss is input to a non-inverted terminal of the comparator 101. When the voltage VA becomes the positive value, the comparator 101 outputs a signal with a low level.
An output terminal of the comparator 101 is connected to one input terminal of the AND circuit 102. A PWM signal is input to another input terminal of the AND circuit 102. When an output voltage of the comparator 101 is a high level, the synchronous rectification transistor M102 switches on/off in accordance with a signal level of the PWM signal. When the output voltage of the comparator 101 is a low level, the AND circuit 102 outputs a low level independently of the signal level of the PWM signal. When the output voltage of the AND circuit 102 is a low level, the synchronous rectification transistor M102 is shut off so as to prevent the reverse current.
However, a minimum duty cycle of the PWM signal which drives the switching transistor M101 cannot be made too small due to circuit delays. When a switching regulator using the soft-start circuit operates under light load conditions at start-up, an overshoot may occur because the output voltage rises too fast to avoid an occurrence of the overshoot using a pulse width of the minimum duty cycle of the PWM signal which prevents the overshoot in a normal load condition. There is no other path but a path through the load to discharge the charge stored in the output capacitor C102, especially on a switching regulator using a rectification diode in stead of the synchronous rectification transistor M102, and the switching regulator shown in FIG. 1 which includes the reverse current protection circuit.
When an on-time of the switching transistor M101 cannot be made short under conditions such as those of a small load current, a large amount of charge may be stored in the output capacitor C102. As a result, an overshoot occurs and the output voltage swings over a maximum voltage. Further, because of the small load current, it takes relatively long to discharge the charge stored in the output capacitor C102. Therefore, the switching transistor M101 may be turned on again by the on-pulse signal of the PWM signal before the output voltage recovers from overshooting.